Field of the Disclosure
The present disclosure relates to a display device, and more particularly, to an organic light emitting diode display. Although the present disclosure is suitable for a wide scope of applications, it is particularly suitable for minimizing a variation in a low potential power voltage on a different position to achieve a uniform luminance in the organic light emitting diode display
Description of the Background
Various display devices have replaced heavier and larger cathode ray tubes (CRTs). Such display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an organic light emitting diode (OLED) display.
In more detail, an OLED display is a self-emission display emits light by exciting an organic compound. The OLED display does not require a backlight unit used in a liquid crystal display and thus has advantages of a thin profile, light weight, and a simple manufacturing process. The OLED display can be also manufactured under a low temperature condition and has a fast response time of 1 ms or less, a low power consumption, a wide viewing angle, and a high contrast, etc. Thus, the OLED display has been widely used these days.
The OLED display includes organic light emitting diodes (OLEDs) converting electric energy into light energy. The OLED includes an anode, a cathode, and an organic emission layer between the anode and the cathode. The OLED display is configured such that the OLED emits light while excitons formed by combining holes from the anode and electrons from the cathode inside an emission layer fall from an excited state to a ground state, and thus displays an image.
However, a large-area OLED display cannot maintain a uniform luminance throughout the entire surface of an active area, on which an input image is displayed, and can cause a luminance variation (or deviation) on different positions. More specifically, a cathode constituting an organic light emitting diode is formed to cover most of the active area, and there is a problem that a power voltage applied to the cathode cannot have a constant voltage value throughout the entire surface of the active area. For example, as a difference between a voltage value at an entrance of the cathode supplied with the power voltage and a voltage value at a position apart from the entrance increases due to a resistance of the cathode, the luminance variation depending on the position increases.
Generally, there is more problematic in a top emission type display device. Namely, in the top emission type display device, because it is necessary to secure a transmittance of a cathode positioned at an upper layer of an organic light emitting diode, the cathode is formed of a transparent conductive material such as indium tin oxide (ITO) and an opaque conductive material with a very thin thickness. In this instance, because a surface resistance of the cathode increases, a luminance variation depending on a position greatly increases corresponding to an increase in the surface resistance.
In order to solve such a problem, a method was proposed to prevent a voltage drop depending on a position by forming a low potential power voltage line including a low resistance material and connecting the low potential power voltage line to a cathode. In the proposed method according to the related art, because the low potential power voltage line was formed on a lower substrate including transistors, one pixel has to further include a connection area of the low potential power voltage line and the cathode in addition to a thin film transistor area and a storage capacitor area. Thus, it was difficult to apply the related art to a high-resolution display including small-sized unit pixels.